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><A
NAME="ECPG-DESCRIPTORS"
>32.10. Using SQL Descriptor Areas</A
></H1
><P
> An SQL descriptor area is a more sophisticated method for
processing the result of a <TT
CLASS="COMMAND"
>SELECT</TT
> or
<TT
CLASS="COMMAND"
>FETCH</TT
> statement. An SQL descriptor area groups
the data of one row of data together with metadata items into one
data structure. The metadata is particularly useful when executing
dynamic SQL statements, where the nature of the result columns might
not be known ahead of time.
</P
><P
> An SQL descriptor area consists of a header, which contains
information concerning the entire descriptor, and one or more item
descriptor areas, which basically each describe one column in the
result row.
</P
><P
> Before you can use an SQL descriptor area, you need to allocate one:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL ALLOCATE DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>identifier</I
></TT
>;</PRE
><P>
The identifier serves as the <SPAN
CLASS="QUOTE"
>"variable name"</SPAN
> of the
descriptor area.
When you don't need the descriptor anymore, you should deallocate
it:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL DEALLOCATE DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>identifier</I
></TT
>;</PRE
><P>
</P
><P
> To use a descriptor area, specify it as the storage target in an
<TT
CLASS="LITERAL"
>INTO</TT
> clause, instead of listing host variables:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL FETCH NEXT FROM mycursor INTO DESCRIPTOR mydesc;</PRE
><P>
</P
><P
> Now how do you get the data out of the descriptor area? You can
think of the descriptor area as a structure with named fields. To
retrieve the value of a field from the header and store it into a
host variable, use the following command:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL GET DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> :<TT
CLASS="REPLACEABLE"
><I
>hostvar</I
></TT
> = <TT
CLASS="REPLACEABLE"
><I
>field</I
></TT
>;</PRE
><P>
Currently, there is only one header field defined:
<TT
CLASS="REPLACEABLE"
><I
>COUNT</I
></TT
>, which tells how many item
descriptor areas exist (that is, how many columns are contained in
the result). The host variable needs to be of an integer type. To
get a field from the item descriptor area, use the following
command:
</P><PRE
CLASS="PROGRAMLISTING"
>EXEC SQL GET DESCRIPTOR <TT
CLASS="REPLACEABLE"
><I
>name</I
></TT
> VALUE <TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> :<TT
CLASS="REPLACEABLE"
><I
>hostvar</I
></TT
> = <TT
CLASS="REPLACEABLE"
><I
>field</I
></TT
>;</PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>num</I
></TT
> can be a literal integer or a host
variable containing an integer. Possible fields are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>CARDINALITY</TT
> (integer)</DT
><DD
><P
> number of rows in the result set
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATA</TT
></DT
><DD
><P
> actual data item (therefore, the data type of this field
depends on the query)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATETIME_INTERVAL_CODE</TT
> (integer)</DT
><DD
><P
> ?
</P
></DD
><DT
><TT
CLASS="LITERAL"
>DATETIME_INTERVAL_PRECISION</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>INDICATOR</TT
> (integer)</DT
><DD
><P
> the indicator (indicating a null value or a value truncation)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>KEY_MEMBER</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>LENGTH</TT
> (integer)</DT
><DD
><P
> length of the datum in characters
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NAME</TT
> (string)</DT
><DD
><P
> name of the column
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NULLABLE</TT
> (integer)</DT
><DD
><P
> not implemented
</P
></DD
><DT
><TT
CLASS="LITERAL"
>OCTET_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the character representation of the datum in bytes
</P
></DD
><DT
><TT
CLASS="LITERAL"
>PRECISION</TT
> (integer)</DT
><DD
><P
> precision (for type <TT
CLASS="TYPE"
>numeric</TT
>)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>RETURNED_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the datum in characters
</P
></DD
><DT
><TT
CLASS="LITERAL"
>RETURNED_OCTET_LENGTH</TT
> (integer)</DT
><DD
><P
> length of the character representation of the datum in bytes
</P
></DD
><DT
><TT
CLASS="LITERAL"
>SCALE</TT
> (integer)</DT
><DD
><P
> scale (for type <TT
CLASS="TYPE"
>numeric</TT
>)
</P
></DD
><DT
><TT
CLASS="LITERAL"
>TYPE</TT
> (integer)</DT
><DD
><P
> numeric code of the data type of the column
</P
></DD
></DL
></DIV
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